DESCRIPTION (adapted from the applicant's description): Congestive heart failure results in substantial structural and functional changes at the level of cardiac myocytes. Preliminary results indicate that the t-tubular network is severely depleted or absent in failing canine and human myocytes. This observation has important functional consequences for excitation-contraction (E-C) coupling (which requires close opposition between surface membrane L-type Ca channels (DHPRs) in the t-tubule membrane and Ca release channels (RyRs) in the SR) and beta-adrenergic signal transduction. The general hypothesis of the proposed research is that sub-cellular remodeling of the t-tubule system and junctional domains results in contractile failure and abnormal beta-adrenergic regulation in failing ventricular myocytes. This general hypothesis will be tested in myocytes obtained from a tachycardia pacing-induced dog model and confirmed on human cells obtained from patients undergoing cardiac transplantation. The 5 specific aims of the proposed research are: 1) characterization of the t-tubule system density in failing and control hearts using 2-photon and confocal microscopy, 2) quantification of DHPRs in failing and control hearts using electrophysiological and biochemical techniques, 3) define the mechanism of uncoupling of the DHPR and RyR in failing myocytes, 4) determine the mechanism of uncoupling of beta-adrenergic receptors and DHPRs, and 5) perform confocal immunolocalization studies of DHPR subunits, RyR, beta-adrenergic receptors and G-proteins.